Write-once optical recording elements in a compact disc (CD) format are commercially available. These elements consist of a polycarbonate substrate containing a continuous tracking groove onto which an organic dye recording layer is solvent coated. A reflective layer and a protective layer are coated in that order over the recording layer. Digital information is recorded in the recording layer with a focused beam of a diode laser operating in the near infrared region of the spectrum.
Recording in optical recording elements, such as a write once recording element, results from the chemical and physical change, in the form of marks, which occurs in and around the dye in the recording layer caused by the recording laser. Such marking causes the light of a read laser, operating at lower powers in the near infrared spectral region, to be diffracted (scattered) affording optical contrast in reflected light. This contrast provides the basis for digital recording.
Mechanisms for marking include dye decomposition with bubble formation as in phthalocyanine dye based media (European Patent Applications 0 451 718 A1, 0 519 419 A2, 0 513 370 A1 and 0 492 508 A1, and Canadian Patent Application 2005520 of Mitsui Toatsu Chemicals, Inc.) and dye decomposition without bubble formation as in cyanine dye based media (e.g. U.S. Pat. No. 4,940,618).
We have found that there is an increased tendency for bubble formation in recording layers containing cyanine dyes as the thickness of the recording layer increases. That is, at thicknesses of about 200 nm and beyond there is a strong tendency for bubble formation to occur.
Bubble formation and dye bleach both result in the lowering of the real index of refraction (N) in the marked area. Lowering of the real index of refraction is the fundamental cause of the diffraction or scattering of the light of the read laser. Bubble formation is not a desired mechanism for marking for the following reasons:
1) Bubble formation may not be uniform and may result in distorted waveforms causing both jitter and non-linearity.
2) Bubble formation may cause the recording layer to fracture resulting in noise and jitter in the read channel.
3) Bubbles in the recording layer cause a physical perforation of the layer impairing the physical integrity of the disc structure.
Any and all of these may impact data integrity and lead to read errors in the recovery of the written data either immediately after writing or after some period of keeping and/or use.